Acoustic emission methods are, inter alia, known from Whitaker et al. Application of acoustic emission to the monitoring and end point determination of a high shear granulation process, Int. J. Pharm., 205, pp 79-91, 2000. Methods of using acoustic emission analysis e.g. on sand powders are known from Esbensen K. et al, Acoustic chemometrics—from noise to information, Chemometrics and intelligent laboratory systems, 44 (1998) 61-76. Methods of using high frequency acoustic emission analysis in fluid beds are known from Tsujimoto H. et al, Monitoring particle fluidization in a fluidized bed granulator with an acoustic emission sensor, Powder technology, 113 (2000) 88-96.